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A lumped parameter technique is described for simplifying the calculation of a thermal transient of a uranium dioxide fuel rod. The calculation is accomplished by evaluating the thermal resistances and capacitances of the pellet and the clad at their average condltions in time and space. Each quantlty is lumped at the middle of lts physical geometry and axial heat conduction is neglected. The equivalent thermal circuit is shown. The technique gives a fairly accurate solution in comparison with other exact methods. (N.W. R.)

A semianalytic model is presented for the prediction of void fractions in subcooled flow boiling at elevated pressures. The model is based on the formation and growth of a bubble boundary layer adjacent to the heated surface at a rate determined by the difference between the imposed surface heat transfer and the heat removal capability of the subcooled liquid core of the flow. The latter heat transfer rate is determined by the analogy between heat and momentum transfer in the liquid employing empirical friction-factor data for low-quality two-phase flow. The analysis is compared to experimental results.

The present subchannel analysis substantially revises the concept of engineering hot-channel factors. Previous design techniques sequentially applied hot-channel factors to the core average enthalpy rise and flow in an attempt to estimate the flow and enthalpy rise in the hottest subchannel of the core. In the present method, a three-dimensional matrix is used to represent the core. Coolant conditions within each matrix element are determined by simultaneous solution of the equations of conservation of mass, energy, and momentum. The behavior of the hot assembly is determined by superimposing the power distribution among the assemblies upon the flow distribution while allowing for flow mixing between assemblies. Another three-dimensional matrix is used to represent the subchannels within the hot assembly. Coolant conditions are obtained from solution of the continuity equations for each matrix element. The flows to and from the hot assembly, computed by the core-wide analysis, are used as a boundary condition. The local variations in power, fuel rod and pellet dimensions, and mixing within the assembly, are superimposed in these calculations in order to determine the conditions in the hot subchannel. The procedure avoids the repetitive application of penalty factors inherent in the use of the simple product of all hot-channel factors.

Subcooled, critical heat-flux test data on refrigerant R-11 flowing vertically in a uniformly heated circular tube is reported. Using the data reported by Coeffield for R-113, a method is presented for obtaining a direct heat-flux scaling factor between refrigerant and water with the same geometry, mass velocity, and local flowing quality when the pressure is adjusted so that the liquid-to-vapor density ratios are equal in both systems.